Transactions of the Korean Nuclear Society Virtual Spring MeetingJuly 9-10, 2020

Seismic Analysis Response of the Base-Isolated APR1400 by Probabilistic Soil-Structure Interaction Analysis

Seung Ryong Han a∗, Hyun Young Chang a aFuture Technology & Strategy Research Institute, KEPCO E&C, 269 Hyeoksin-ro, Kimcheon, GB 39660

*Corresponding author: hoanung@kepco-enc.com

1. Introduction

The recent earthquakes in Gyeongju and Pohang have arised importance of safety of domestic nuclear power plants (NPP) against strong earthquakes. Seismic performance of NPPs is identified with performance goals which are based on probabilistic approaches [1]. For seismic design of the base-isolated nuclear power plants, probabilistic and nonlinear analysis are essential in consideration of multiple sets of ground motions and nonlinear analysis considering nonlinear behavior of seismic isolators and Soil-Structure Interaction (SSI). In this study, seismic responses of the base-isolated APR1400 are calculated by probabilistic and nonlinear seismic analyses on the 30 cases of SSI analyses with 10 sets of time histories. The seismic analyses are performed on time-domain with ANSYS. Effects of variability of structural stiffness and damping, soil properties, and seismic time histories with consideration of SSI are considered in the SSI analysis and the results are compared with one another. The probabilistic SSI analyses are performed with Latin Hypercube Simulation (LHS) and Bootstrap random resampling [2]. The results of the probabilistic seismic analysis show that the seismic responses are somewhat different from those of deterministic seismic analysis.

2. Methods and Results

In this section, it is described how the probabilistic SSI analysis model is set up. An analysis model with seismic isolator design, seismic input motions, and concept of the probabilistic SSI analyses are included in detail. Also, the results of the probabilistic SSI analyses are shown in the following.

2.1 Analysis Model

A finite element model of the base-isolated ARP1400 consists of Reactor Containment Building (RCB), Reactor Containment Internal Structures, Reactor Coolant System, Auxiliary Building (AB), and a seismic isolation system. Except the basemats of upper and lower structures, all upper superstructures are modeled with beam elements which have linear properties. 454 isolators are installed in the analysis model connecting the upper basemat of superstructures to the lower basemat of bottom. Lead-plug Rubber Bearings (LRB) are representatively used in this study. The idealized

bilinear material properties of isolators are schematically shown in Fig. 1.

Fig. 1. Bilinear material properties of seismic isolators

The finite element model of the base-isolated ARP1400 is shown in Fig. 2. The variation of the soil properties for SSI analysis is consistent with the soil properties used in the generation of seismic input motions [3].

1

XY

Z

force vs displacement relationship

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ELEMENTS

MAT NUM

1

XY

Z

force vs displacement relationship

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ELEMENTS

MAT NUM

Fig. 2. Finite element model of the base-isolated APR1400

2.2 Seismic Input Motion

Transactions of the Korean Nuclear Society Virtual Spring MeetingJuly 9-10, 2020

Seismic input motions are developed by enveloping Korean seismic design response spectrum of control motions (CMS+) based on the design response spectrum of the US NRC Regulatory Guide (RG) 1.60 which is enriched to the higher frequency range. The spectral matching for these input motions is performed at multiple damping ratio targets. The spectrally matched input motions are scaled to 0.5g of peak acceleration for Safe Shutdown Earthquake. Fig. 3 shows the final input motions which envelop target spectrums and satisfy the Standard Review Plan (SRP) 3.7.1 of US NRC [4].

10-1

100

101

102

10-2

10-1

100

101

frequency(Hz)

Sa (

g)

0.5, 2, 5, 7, 10% dampings in the horizontal direction 1

10-1

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0.5

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Frequency (Hz)

Sa (

g)

5% damping selected among multiple dampings in the horizontal direction 1

Target

Matched

Mean

Target

Target×1.3

Target×0.9

Matched

Mean

Fig. 3. Seismic design response spectrums and multiple seismic input motions

2.3 Probabilistic Soil-Structure Interaction Analysis

Fig. 4. Comparison between horizontal spectral accelerations of deterministic and probabilistic analyses

Fig. 4 and Fig. 5 show spectral accelerations of the deterministic and probabilistic SSI analysis results at 78ft and 333.5ft floors of RCB.

Fig. 5. Comparison between horizontal spectral accelerations of deterministic and 80 percentiles of probabilistic analyses

3. Conclusions

The difference between the deterministic seismic analysis without SSI and the probabilistic seismic analyses with SSI for the base-isolated APR1400 is not obvious. Therefore, we can consider the results of the deterministic seismic analysis without SSI for seismic design of the base-isolated nuclear structures based on the current Standard such as ASCE/SEI 4-16 if we apply a little of widening and scaling on the seismic responses [5].

REFERENCES

[1] ASCE/SEI 43-05, “Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities,” American Society of Civil Engineers/Structural Engineering Institute, 2005. [2] American Society of Civil Engineering (ASCE), “Seismic Analysis of Safety-Related Nuclear Structures and Commentary”, ASCE 4-16, March 2017. [3] U.S. Nuclear Regulatory Commission (NRC), “A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion”, Regulatory Guide 1.208, 2007. [4] U.S. Nuclear Regulatory Commission (NRC), “Design Response Spectra for Seismic Design of Nuclear Power Plants,” Regulatory Guide 1.60, 1973. [5] KEPCO E&C, “Probabilistic Seismic Analysis for Nuclear Power Plants with Seismic Isolation System”, Final Report, KEPCO E&C/18–TRA62-01, 2018.